bootstrap: Set ECDSA as the default SSH key algorithm

Motivation: RSA SHA-1 SSH keys are no longer accepted by GitHub https://github.blog/2021-09-01-improving-git-protocol-security-github/.
Given this we are switching the default from RSA to ECDSA for `git`, `github` and `gitlab` variants of `flux bootstrap`.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/bootstrap.go

@@ -140,7 +140,7 @@ func NewBootstrapFlags() bootstrapFlags {
 	return bootstrapFlags{
 		logLevel:           flags.LogLevel(rootArgs.defaults.LogLevel),
 		requiredComponents: []string{"source-controller", "kustomize-controller"},
-		keyAlgorithm:       flags.PublicKeyAlgorithm(sourcesecret.RSAPrivateKeyAlgorithm),
+		keyAlgorithm:       flags.PublicKeyAlgorithm(sourcesecret.ECDSAPrivateKeyAlgorithm),
 		keyRSABits:         2048,
 		keyECDSACurve:      flags.ECDSACurve{Curve: elliptic.P384()},
 	}